Engines have used various forms of variable cam timing to improve engine operation over a variety of speed/load conditions. Further, hybrid vehicle systems may use variable cam timing to improve overall vehicle operation.
One approach to such a system is described in Japanese SAE paper 9739552. This system described a hybrid electric vehicle (HEV) using an Atkinson engine with intake variable cam timing enabling late intake valve closing during shutdown and cranking (120 deg after BDC). The late intake valve closing may be used to reduce engine vibration during engine restarts.
However, the inventors herein have recognized a problem with such an approach. Specifically, Atkinson-cycle engines typically suffer a fundamental disadvantage of poor torque at low to medium engine speeds. The reduced peak torque levels may then lead to secondary problems with noise, vibration, and harshness (NVH) and fuel efficiency because higher engine speeds are required to produce sufficient power in real customer driving.
Thus, in one approach, the above issues may be addressed by a system for a vehicle, comprising: an internal combustion engine coupled in the vehicle, the engine having at least one cylinder with an intake and exhaust valve, where the opening and closing timing of the intake valve is adjustably retardable and the opening and closing timing of the exhaust valve is adjustably retardable, during engine operation; and an energy conversion device coupled in the vehicle capable of selectively supplying and absorbing torque during vehicle operation.
In this way, it is possible to obtain improved starting operation, for example by utilizing both intake and exhaust retard. Further, it is also possible to obtain improved torque output during low to mid engine speeds. In other words, late intake/exhaust valve timings may be used to reduce fresh air pumped through the engine during engine shutdown and cranking, thereby reducing oxygen flow to the catalysts in the exhaust. Further, such operation also may reduce NVH during engine starting (cranking) and/or shutdown operation. However, by having variable intake and exhaust valve timing retard, wide-open throttle torque penalties such as in the Atkinson cycle are reduced, and it is actually possible to obtain some torque and power improvement.
Additionally, using both intake and exhaust valve timing retard reduces issues with late intake valve opening in a non-Atkinson cycle engine with variable intake valve timing retard. For example, in such a case, the late intake valve opening may increase noise and vibration, and again the valve timing adjustments may not assist in improving wide-open throttle torque output. Thus, more advanced timings may be used at least during some higher torque output conditions to better take advantage of the hybrid propulsion system and obtain better overall vehicle performance during real world driving conditions.
Furthermore, by using both intake and exhaust valve timing retard it is possible to obtain improved fuel efficiency and feedgas emissions at part throttle operating conditions, for example.